1. Field of the Invention
This invention relates to a camera which divides a field of view into a plurality of areas so as to perform a light measuring operation.
2. Description of the Related Art
There have been suggested various kinds of light measuring devices which divide a field of view into a plurality of areas, output luminance value signals of the respective areas, and give a correct exposure to a photographed image plane by using the plurality of luminance value signals.
For example, Japanese Utility Model Publication No. Sho 51-9271 suggests a light measuring device which obtains an arithmetical average value of the maximum value and the minimum value of outputs from a plurality of photoelectric conversion elements.
Furthermore, Japanese Patent Publication No. Sho 63-7330 suggests a light measuring device which divides a field of view into a plurality of areas, performs light measuring operation for a center area and a plurality of external areas arranged so as to surround the center area, regularizes the luminances of the plurality of areas by a standard value set between the maximum value and the minimum value of the luminances of the areas, classifies the field of view based on the regularized outputs, and calculates a light measuring value according to the classified outputs.
Although the above prior art gives little consideration to the arrangement of a main object in a photographed image plane, a light measuring device, in which the arrangement of the main object in the, photographed image plane is much considered, has also been suggested on the assumption that it is mounted in a camera including an automatic focus detecting device and so on.
For example, U.S. Pat. No. 4,912,495 suggests a light measuring device which regards a center portion of an image plane as a position where a main object is often placed, divides a field of view into a plurality of areas including at least a plurality of areas concentrically disposed around the center portion of the image plane, can precisely find a luminance of the main subject by selectively summing luminance value signals of the concentric areas by assuming the size of the main object based on information on magnification. This U.S. Patent also suggests changing the formula for operating a light measurement value which calculates the luminance of a background besides that of the main object and utilizes the difference between these luminances, so that a correct exposure is given to the main object and furthermore, an exposure suitable for the photographic conditions can be given if the main object is small.
U.S. Pat. No. 4,786,935 also suggests a light measuring device which regards a center portion of an image plane as a position where a main subject is often placed, divides a field of view into three areas, that is, a center area of the image plane, an outer area thereof and a further outer area thereof, simultaneously determines an approximate size of the main object and photographic conditions based on luminance value signals of these plurality of areas and the difference between luminance value signals of the adjoining areas, and provides a correct exposure. In this U.S. Patent, since an approximate size of the main object is determined by using the difference of the luminance value signals, it is more difficult to depend upon an actual size of the main object and a correct exposure can be obtained more stably compared with a device which assumes the size of the main object based on magnification.
In the two devices of the prior art quoted herein, the center portion of the image plane is regarded as a position where the main object is often placed because a focus detecting area of a camera having an automatic focus detecting device is generally set in the center of the image plane. On the other hand, an automatic focus detecting device having a plurality of focus detecting areas has been recently suggested. In a camera having such an automatic focus detecting device, the main object is easy to be placed in each of the plurality of focus detecting areas (therefore, also in specific areas besides the center portion of the image plane).
A light measuring device suitable for use in such a camera having a plurality of focus detecting areas is suggested in Japanese Laid-open Patent Application No. Hei 1-202720. This light measuring device utilizes a suggestion by Japanese Laid-open Patent Application No. Sho 62-203022 that the luminance of a main object may be determined based on information on the above-mentioned magnification, and performs an operation to change the weight of a certain light measuring area and other light measuring areas based on information on a selected focus detecting area and information on a focusing state of other focus detecting areas when the selected area is in an in-focus state, besides information on the magnification and information on the focal length of a picture-taking lens, finds a precise luminance of the main object and outputs the luminance as a light measurement value for natural light photographing.
Among the above conventional light measuring devices, only the Japanese Laid-open Patent Application No. Hei 1-202720 discloses a light measuring device suitable for use in a camera having a distance measuring device capable of separately measuring distances of a plurality of points in a field of view. Although this light measuring device additionally has a concept that the focusing states of a plurality of light measuring points are considered, the basic technical concept thereof is the same as that of the Japanese Laid-open Patent Application No. Sho 62-203022 which changes the area in the field of view to be considered in a light measurement value calculating operation in accordance with magnification so as to find a precise luminance of the main object.
Therefore, the disadvantage that the luminance of the main object is incorrectly measured and it is difficult to obtain a correct exposure if an actual size of the main object is different from the size which is initially assumed, is also succeeded. Furthermore, in such a technical concept that the area of the field of view to be considered is changed in a light measurement value calculating operation in accordance with the magnification, if the magnification becomes low, a discontinuous point in determination of photographic conditions, that is, whether a main object is regarded as a narrow area or almost the whole field of view, arises. Furthermore, if the difference between the luminance of the area to be measured and that of the other areas is large, the light measurement value is greatly changed by the result of the determination. In such a light measuring device, it is essential to use information on the focal length of the picture-taking lens and information on the object distance for the light measurement value calculating operation besides the luminance signals as outputs from the light measuring device, and, particularly in a single-lens reflex camera, devices for transmitting various kinds of information, processing signals and so on are complicated.
The above prior art, Japanese Laid-open Patent Application No. Hei 1-202720 discloses the concept that the existing range of the main object is assumed in consideration of the focusing state of a plurality of points to be measured and the weight of the luminance signal of the area of the field of view corresponding to each of the points to be measured. Such a concept is effective when the magnification is high, the main object occupies most parts of the photographed image plane and when the distances between a plurality of distance measuring points in the photographed image plane are short. However, in a general case, such as when a plurality of distance measuring points are arranged at some intervals in the photographed image plane and the main object does not occupy so large a part of the photographed image plane, since a background intervenes between a plurality of distance measuring points in an almost in-focus state, in particular, if the luminance difference between the main object and the background is large, the effect is lowered.